1. Field of the Invention
This invention is directed to a modified method of preparing blown films comprising a major proportion of polyethylene, at least one additive, such as a colorant or an antiblocking agent, and a minor proportion of a second polymeric resin.
2. Description of the Related Art
Linear low and medium density polyethylene copolymers (LLDPE), and linear high density polyethylene (HDPE) are known materials which are widely commercially available. LLDPE is described by Anderson et al., U.S. Pat. No. 4,076,698 which is incorporated herein by reference. HDPE is described, e.g., in KIRK-OTHMER ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY, Third Edition, Vol. 16, pages 421-433, John Wiley and Sons, New York (1981), incorporated herein by reference. A commercially important use of LLDPE and of HDPE is in making bags which are conventionally prepared from films by blown film extrusion. While such films have relatively good properties, they often exhibit low stiffness and have relatively weak MD tear strength characteristics. Accordingly, it has been proposed to improve MD tear strength characteristics of such films by incorporating a small proportion of a second polymeric resin, e.g., aromatic polymeric resins, such as polystyrene or poly(para-methylstyrene), into the polyethylene film, see Canterino et al, U.S. Pat. No. 4,579,912, the contents of which are incorporated herein by reference. The films of Canterino et al are made by blending separate batches of the polyethylene resin, the aromatic polymeric resin and usually at least one conventional additive, such as a colorant, an anti-blocking agent, or a processing aid, outside of the extruder, or introducing separate streams of the polyethylene resin, the aromatic polymeric resin and the additive(s) into the extruder, and subsequently blow-extruding the film. The resulting film, when viewed under an electron microscope, exhibits a continuing phase of polyethylene containing discrete, distinct microdomains of the aromatic polymeric resin.
To prevent the preferential incorporation or agglomeration of the additive or additives into the microdomains of the aromatic polymeric resin, it was thought necessary to admix the additive or additives with the polyethylene resin, and then combine the resulting mixture with the minor proportion of the aromatic polymeric resin. The polyethylene resin was also used as the carrier for additive(s) because it is customary in the industry to use the predominant resin as the carrier for additives in the preparation of films. Alternatively, as discussed above, separate streams of the polyethylene resin, the aromatic polymeric resin and the additive or additives were introduced into the extruder.
Both methods of feeding the components used for manufacturing the blended film of polyethylene and aromatic polymers were cumbersome and rendered the control of the amount of the additive or additives, and of the second polymeric resin introduced into the extruder, difficult and time consuming. Each of the additive and polymeric streams had to be monitored and controlled individually and the amount of each individual component in the film also had to be determined individually.
Accordingly, it is a primary object of the present invention to simplify the method of preparing a blown polyethylene film from a major proportion of a polyethylene resin, a minor proportion of a second polymeric resin and at least one additive.
It is an additional object of the present invention to provide a convenient yet simple method of monitoring the amount of the second polymeric resin incorporated into the polyethylene film.
Additional objects of the invention will become apparent to those skilled in the art from the following description of the invention.